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September 1, 2022

Welcome back to Head Shepherd. I'm pretty excited this week to have Dr. Johan. Johan's had a long career in, really studying a lot of welfare traits and a renowned geneticist across Australia. Welcome, Johan.

Thank you, Mark

We've got a lot to talk about. I’ve followed your work for a long time and we used to work together for a while. I’ll start with a bit of your background from studying in South Africa through to moving to Perth eventually.

Yeah, I grew up in South Africa and I went to various universities and then I started working at the Animal and Dairy Science Research Institute. I worked there for about 17 years I think. Then I came here to Australia in 1993.

Fantastic. I know when I was studying in my PhD days, a lot of great animal science came out of South Africa. A lot of great animal scientists there. Back in that area when you were there it was a lot of the foundational work done in South Africa.

I was mostly involved in the high fertility breeds, like the Finnish Landrace and Romanoff and then also meat quality/meat science. My colleagues at the time were implementing the breeding evaluation technologies that provided breeding values for the different traits for the different breeds in South Africa. So that was an exciting time. But I was mostly involved with introducing fertility genes into the populations, and that's where I got involved in developing a white wool sheep breed for the intensive regions in South Africa.

Then we developed a very fertile composite. We sent some of these animals out to farms to see how the farmers looked after them. But of course they needed denser conditions and those denser conditions were on irrigated pastures. And under those conditions, the farmers' management skills were just not up to scratch. The worms just had a feast on those pastures with the sheep.

That's when I realized that you can have the best and most fertile sheep, the most productive sheep, but if they're not fit and they're not resistant to worms, you will not have a very profitable sheep population.

That's actually why I got involved in breeding for fitness and breeding for worm resistance in particular. And that was the reason why I came to Western Australia to join the team. They started off four years before I arrived here. They had started their new project where John Townsend was involved with developing the genetic technologies to breed sheep that are resistant to worms.

That's good to hear the back story. Norm Adams was a great collaborator of yours, and was also my PhD supervisor, and he was very interested in all that sort of fitness area as are you.

Norm was a great collaborator. That part of fitness was a big issue for us. And it was good for Norm to come on board at that time in order to look at this whole fitness issue, especially the issue of fat. Because in the Merino, we knew that there's a negative relationship between wool cut and fertility. It was Norm that took that one further and of course you came further in your PhD.

Don Carlson has been a great colleague of yours over many years. Could you outline the Rylington flock and what it set out to achieve and then and when it changed gears to stop just selecting on worms and actually start putting those traits into a more productive sheep?

There was a time when John started off with a project; Ivermec was not available and most of the worms here in Western Australia, in Australia all over, had become resistant to the available drenches. So there was a big push to breed sheep that's resistant to worms, and especially in Western Australia with this unique environment of very dry, hot summers and with cold winters.

And so John, with some support from AWI and the support from a lot of breeders who contributed a small number of ewes, set up the foundation population. He sourced rams from different sources that were exposed to some form of selection for worm resistance or natural selection. The flock that he sourced rams with an active selection was from CSIRO and also from Armidale.

At the start he measured fecal worm egg count as the indicator trait to see whether that can be used to breed animals who are resistant. We didn't know, in this environment, when was the best time to measure worm resistance. So John measured FEC on a monthly basis for the first three or four years.

When I came on board they had all of the data, and then I started to analyse it. It was clear that the best time in this environment to identify genetically resistant animals is when the herd immunity with the trait is the highest.

And in our environment, it was shown to be about two months after the break of the season. We found that the heritability was actually quite high during that time, whereas during the summer although there were egg counts, the heritability of the trait was zero; it was really not of any use to measure worm egg counts then.

It was a unique program where we could start monitoring these different sheep mobs that we want to select from, after the break of the season, and then about two months after. That's normally the time when the worm egg count reaches the desired level- in our environment that's around 500 eggs per gram - that is a minimum with the target that we want. Then we used the data to estimate the heritability and to estimate breeding values. And that's the data that we used to select animals for low worm egg count.

And that had quite a significant effect. It reduced on average from 1988 to 2006, by 2.5% per year- it was quite a significant decline. 

But we were not sure whether the animals were just restricting the worm's ability to lay eggs or does it have an impact on the worm? So we had a trial where we actually drenched susceptible control sheep. We put them in the Animal House and drenched them with 20,000 larvae, three times a week then we measured the worm egg count. All the animals got exactly the same amount of larvae, and we measured worm egg counts at the end of the trial. The animals were then slaughtered and we counted the worm eggs. There was a dramatic reduction in the amount of worms in the resistant animals, especially for Trichostrongylus, but less so for Teladorsagia. 

But it was clearly shown that if you select low worm egg counts, it has a dramatic effect on reducing the ability of the worm to establish and also to lay eggs. So the next place that we were interested to see was whether these animals were really more productive, if they are resistant. And that's with the next phase of what we've done.

Yeah, it's very cool stuff. So did you end up working out what the sheep is actually doing to restrict the larvae? Is it obviously mounting some sort of immune response?

Yeah, it is. It is actually quite a complex area. We have been looking at different areas to see how the immune system responds to worms. We have never really pinpointed it.

We have looked at immunoglobulins, IgA,IgE, IgG and IgM. The only one that is sort of having some sort of effect is IgA. We also looked at all the white blood cells, their lymphocytes, neutrophils, the basophils and those cells. But there was no difference there. The only one that has got an effect were the eosinophils. When the animal is infected with the worms, the eosinophils are increased. But there was no difference between the selection line and the control lines with the eosinophils. But all the sheep that were affected had increased eosinophils. So eosinophils play a part, but it is not eosinophils that cause a difference between selection lines. 

And as I say, the only one trait was just the IgA, which had an effect. But they would have liked to do some more work on that front, but we just weren't too successful to get money to do that.

It’s interesting and we often get asked about the footrot work we've been doing -what the cause is- but we have no idea. We know that we can select them to be more resistant to it. And we know that it's across a big chunk of the genome and has lots of genes involved, but that's about as close as we get. We've gotten probably as close as we might ever get.

We had a big experiment recently with a PhD student. They looked at the worm resistance and also the propensity to scour with dags. There was a fear at one stage that if you select for a low worm egg count then there was an increase in the formation of dags, in our case. And therefore one has to select for both traits to prevent that, so you select one that is resistant but with low dags. And we wanted to look and see what are the underlying factors, and to see which genes are expressed.

We generated extreme groups involving high and low WEC and high and low dag sheep. So it was a two by two with these four groups involved. We wanted to see what the immune profiles of these four groups were. And we had some RNA studies to see which genes are expressed for these different types.

There's not single genes involved, it's all a lot of genes, like typical polygenic traits, a lot of genes with small effects. All that we can get at this stage is that there's a huge variety of genes playing a part. But how, we're not really sure.

And on the other side, the low worm egg counts, and the low dags- the resistant groups, we couldn't really find out actually what makes them different from the work we've done. With a lot of genes involved… with strange genes… with how they work together, we're not sure.

So you're right, we don't really understand this- how the resistance in animals works and how they protect themselves against worms.

Dag score as per the neXtgen agri visual appraisal guide

Yeah. And while we don't know how they work, we know it works unbelievably well. We take it for granted now that we can select for worm egg counts, or for dag. But after all that foundational work, when you see a very low dag breeding value animal come through, or its progeny come through clean, in a high dag environment, it's just an amazing, amazing thing to see. Especially when every other sheep's full of dags. So we know it works. We just don't know how.

Yes, absolutely. You see these large numbers of animals with dags, and then you get these other animals with no dags in the same environment. And that's the ideal type of animal. And fortunately, genetics works well. 

We don't always have to understand exactly what's going on there, but it's nice to know. But fortunately we just focus on the genes or the traits of importance that we can select with those genes affecting and how it works it's not always that important. The important thing is to have a result where you have resistant animals that are an easy care type of animals.

I have a vague recollection of a trial that you and John ran. I guess one of the problems with breeding for resistance is if you run them in a flock of susceptible animals for worms, they're busy fighting everyone else's worms. I think you ran a trial where you put them out on their own farm. A trial where you could sort of see the benefits of worm resistance, because they weren't having to deal with the rest of the sheeps' worms. Is that accurate?

Yeah. It was quite an extensive trial that lasted for eight years. The big issue is that, to test whether the animals are really more productive, or if there is a difference with reduction, you have to run them separately. Otherwise the resistant animals pick up all the worms and act as a vacuum cleaner for the susceptible animal. So the susceptible animals get a free ride. So you have to split the susceptible or the control animals from the resistant animals. 

We had these elaborate trials where we replicated the little paddocks for each treatment where we then split them. There's never any contamination across different treatment groups. After the lambs weaned, they’d go into their own little paddocks up to Hogget age where we would evaluate them and see how they perform. The ewes stay in their little paddocks all the time. We had to generate these replacements, these resistant animals, from their own rams so they on average were only 20% more resistant than the control after 8 years.

But, that 20% resistance, that lowered WEC, resulted in the weaned lambs of the resistant group to be 1.5 kilograms heavier at weaning. They were also one kilogram heavier at hogget age. They had to drench them 60% less because of a 20% increase in worm resistance. 

You can really extrapolate, as a rule of thumb from our environment, for every percentage increase in worm resistance, you will get that 1% decrease in the need for drenching, which was quite a nice outcome. 

Also we didn't have any found difference in fleece weight, but we did find that the resistant animals, although they cut the same amount of wool, actually had finer wool- about three or four micron finer. And the reason for that is that we didn't drench all the animals at the same time.

We ran them as little farmlets and only when the animals reached a target of 500 eggs per gram, did we go in with a drench gun and drenched that mob only.

So with the resistant animals, we would push out drenches for as long as we could, whereas the susceptible animals would require a drench much earlier. So the resistant animals were stressed much harder and subjected to more worms than the control animals. And that is the reason why they were finer, but at the end when they were drenched, they picked up much quicker. If there was a reduction in wool cut, that's the compensatory growth off of that and just cancels it out. So at the end of the day, they produce the same amount of wool. They were finer than resistant animals, but they were also heavier and that was a really nice outcome.

And you have of course, you have to drench them much less.

I often get asked, we've now got industry leading animals out there that are, on an ASBV basis, around -80, -90, is that achieved by zero drenching or is that very, very limited drenching when you get ones that are that worm resistant.

Yes. Well in our case, the way we run them on their own, we found it very hard to get the egg counts up to an acceptable level where we can measure WEC to identify which animals are resistant. That was very, very difficult.

The animals were really resistant, we didn't have to drench, so we had to really do something special. We had to create paddocks, contaminated paddocks, where we can put these young animals and yet we couldn't increase the WEC at hogget age, it's just not possible. They were always low. So we shifted our focus to measuring WEC at weaning and that's when they're much more susceptible. But before that we really made use of contaminated paddocks of other sheep that were resistant to that will be a challenge and that was the only way we could do it.

Then in 2008, we included the Rylington flock in with our breech strike flock. The breech strike flock was not as resistant as the Rylington flock. The breech strike flock contaminated the paddocks very easily. But it then became clear that the Rylington sheep must protect the breech strike sheep, because we had difficulty across the whole flock to get the worm egg counts up whereas previously it wasn't the problem with the breech strike animals on their own

Such an awesome problem to have. And I guess we're going to see more of that. Luckily, and we've got among our clients at least, a massive interest in breeding for higher welfare and low chemical inputs, which is exactly what we're talking about. You did a lot of work around fecal consistency and the importance of correcting egg count for fecal consistency. What did that show?

There was a general feeling, especially among the parasitologists, that when the animals are infected with worms it increases the animals propensity to scour more. Therefore, wet dags will sort of dilute the WEC and therefore there was a feeling that one should correct the feces for the amount of moisture to get a much more accurate indicator of WEC, at this “standard” constant moisture content.

So I looked at this. I used our sheep and we dried the feces of the animals and measured the fecal WEC of the different samples. And we then saw- what is the impact, if you dry it that level, on the breeding value of the animals?

So we compared the fecal WEC of normal feces, irrespective of the higher moisture content and then did a standard one. The genetic correlation between those two traits was virtually, 0.99 It was virtually exactly the same thing. We plotted it and it was virtually a straight line and it had made no impact whatsoever on the ranking of the animals. There was a sort of a scattering at the high end of the dags group, the more moist feces but not on the dry side.

And those are the animals that you want: the animals with a low moisture content and the ones that are resistant.

So that was a nice outcome. You don't want to use any of those animals who have a high moisture content in any case. So you're focusing on these dry animals and that was really amazing.

Sophie Barnes
Article by:
Sophie Barnes

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